Input buffers are commonly used in a wide variety of integrated circuits. Buffers generally perform a number of advantageous functions when used in digital circuits. For example, buffers generally provide a high input impedance to avoid excessively loading circuits to which they are connected, and they have a low output impedance to simultaneously drive electrical circuits without excessive loading. Buffers can condition the signals applied to internal circuits so that the internal signals have well-defined logic levels and transition characteristics. Buffers are used, for example, to couple command, address and write data signals from command, address and data buses, respectively, of memory devices, including dynamic random access memory (“DRAM”) devices.
There are also several types of input buffers. For example, there are single ended input buffers in which a single input signal is applied to the buffer to cause the buffer to transition when the input signal transitions through predetermined voltage levels. Single-ended input buffers may also be used to compare the input signal to a reference voltage so that when the input signal transitions through the reference voltage the output of the buffer also transitions. Differential input buffer circuits are useful in digital circuits for determining whether an unknown input voltage is either above or below a fixed reference voltage. A conventional differential input buffer 100 is shown in FIG. 1 that includes a pair of differential amplifiers 101, 103, and an output coupled to an inverter 140. The amplifiers 101, 103 are connected in parallel between a PMOS transistor 102 that is coupled to a supply voltage VCC and an NMOS transistor 104 that is coupled to ground. When enabled by an active low signal EN_, the supply voltage VCC supplies a current through the PMOS transistor 102 to a node 105. As a result, a constant current is provided to the amplifier 101 and a PMOS transistor 122 coupled to the amplifier 103. Similarly, the supply voltage VCC is directly coupled to the gate of the transistor 104 such that a constant current is coupled from a node 115 through the transistor 104, thereby drawing current through an NMOS transistor 110 coupled to the amplifier 101 and to the amplifier 103. Therefore, the transistor 122 functions as a current source providing constant current to amplifier 103, and transistor 110 functions as a current sink to discharge a constant current from amplifier 101.
The two differential amplifiers 101, 103 have essentially the same components, but are complementary configured with respect to each other. The differential amplifier 101 includes a pair of PMOS transistors 116, 118 whose gates are coupled to each other and to node 105, and further coupled to the drain of the PMOS transistor 118. The transistors 116, 118 are coupled to each other in a current mirror configuration so they both have the same gate-to-source voltage. As a result, the transistors 116, 118 have the same source-to-drain resistance. The drains of the transistors 116, 118 are coupled to the drains of two NMOS transistors 112, 114 respectively, and receive an input signal VIN and a reference signal VREF at their respective gates. When the magnitude of the VIN signal is at ground potential, the transistor 112 is turned OFF. As a result, an output node 108 at which an OUT_signal is generated is driven high through the PMOS transistor 116. An inverter 140 having an input coupled to the node 108 thus generates a low DIFF_OUT signal. When the magnitude of the VIN signal is at VCC, the transistor 112 is turned ON with a significantly higher gate-to-source voltage than the gate-to-source voltage of the PMOS transistor 116. As a result, the resistance of the transistor 112 is significantly lower than the resistance of the transistor 116. The voltage at the output node 108 is therefore low enough so that the inverter 140 outputs a high DIFF_OUT signal. As the magnitude of the VIN signal passes through the magnitude of the VREF signal, which is typically VCC/2, the NMOS transistors 112, 114 have the same gate-to-source voltage and hence the same resistance. Furthermore, the NMOS transistors 112, 114 will have the same gate-to-source voltage as the PMOS transistors 116, 118. If the NMOS transistors 112, 114 have the same electrical characteristics as the PMOS transistors 116, 118, the PMOS transistors 116, 118 will then have the same resistance as the NMOS transistors 112, 114. In such case, the OUT_voltage will be equal to VCC/2. Therefore, decreasing the magnitude of the VIN signal increases the resistance across the transistor 112, reducing the current through the transistors 112, 116 to cause the magnitude of the OUT_signal to increase. Conversely, increasing the magnitude of the VIN signal decreases the resistance across the transistor 112, increasing the current through the transistors 112, 116 to cause the magnitude of the OUT_signal to decrease.
The amplifier 103 includes components that are the same as the amplifier 101, and thus for the sake of brevity, the components to the amplifier 103 will not be described in detail. The amplifier 103 has a topology that is complementary to the topology of the amplifier 101 so that the gates of NMOS transistors 128, 130 are coupled together in a current mirror configuration so that both transistors 128, 130 have the same resistance. As the magnitude of the VIN signal increases, the resistance of the PMOS transistor 126 increases to cause the magnitude of the OUT_signal to decrease. Conversely, as the magnitude of the VIN signal decreases, the resistance of the PMOS transistor 126 decreases to cause the magnitude of the OUT_signal to increase.
When the magnitude of the VIN signal decreases below VT, where VT is the threshold voltage of the NMOS transistor 112, the transistor 112 is turned OFF and thus no longer responds to changes in the magnitude of VIN. Similarly, when the magnitude of the VIN signal increases above VCC-VT, where VT is the threshold voltage of the PMOS transistor 126, the transistor 126 is turned OFF and thus no longer responds to changes in the magnitude of VIN. Thus, the buffer 100 can operate at all values of VIN from 0 to VCC, but only one amplifier 101 or 103 is operable with the magnitude of VIN below VT or above VCC-VT.
When the difference between VIN and VREF is small, such as when VIN transitions through VREF, the integrity of the input signal can be easily compromised by a number of interferences, such as improper bus termination, reflections, signal noise, and VREF noise. These factors can result in false switching of the buffer 100 as shown in FIG. 2. For example, due to the presence of noise, the VREF signal may fluctuate about its predetermined value, such as VCC/2. As the VIN signal approaches the VREF signal, the noise interference on the VREF signal may overlap the VIN signal such that a slope reversal 205 occurs, where the buffer 100 detects VREF to be greater than VIN, when in fact VIN is intended to be greater than VREF, but may not be due to noise and or signal reflections. Consequently, the buffer 100 may falsely switch its output, thereby generating an incorrect response to the input signal and causing delays or resulting in errors to the overall operation of the system or component that relies on the buffer 100.
Therefore, there is a need for a low current input buffer that reduces false switching in the presence of noise due to input signal slope reversals, and restores signal integrity.